In the fields of optical communication, optical information, and optical measurement, semiconductor light receiving elements are often used as photoelectric conversion elements. A principal factor that limits response speed of photoelectrical conversion in the semiconductor light receiving element is the time constant of a circuit, determined by the product of load resistance and electrical capacitance created by a depletion layer, and carrier travel time required for a carrier to pass through the depletion layer, and in order to improve high speed responsiveness of the semiconductor light receiving element, it is desired to decrease the time constant of the circuit and the carrier travel time.
As a means of decreasing the time constant of the circuit, for example, a means can be noted of reducing light receiving area and decreasing element capacitance. In a semiconductor light receiving element described in Patent Document 1, for the semiconductor light receiving element, which has a structure in which an i-type semiconductor light absorbing layer and a p-type semiconductor layer are stacked in this order on an n-type semiconductor substrate layer, by forming a plurality of opening regions reaching from the p-type semiconductor layer to the i-type semiconductor absorbing layer, effective pn junction area is reduced without reducing light receiving area, and capacitance reduction is aimed at.
Furthermore, in recent years, there have been many kinds of attempts in order to increase speed and realize high efficiency in the semiconductor light receiving element, using a metal surface plasmon or photonic crystal structure, as compared with the conventional situation (for example, refer to Patent Documents 2 to 4).
In a description in Patent Document 2, a Schottky photodiode that uses surface plasmons includes: a conductive film having a hole of a diameter smaller than the wavelength of incident light, and a periodic structure that causes a resonance state on a film surface by the surface plasmon that is excited, by the incident light on the film surface of the conductive film provided around the hole; and a semiconductor layer arranged to be in contact with the conductive film in the vicinity of the hole of the conductive film. The light incident on the periodic structure undergoes photoelectric conversion, by exciting surface plasmons, causing the resonance state with the surface plasmons, and generating near-field light at an interface between the conductive film and the semiconductor layer.
A surface plasmon strengthening photovoltaic device described in Patent Document 3 has a first metallic electrode, having a surface on which incident light is illuminated and a surface which is not illuminated, at least one of the illuminated surface and the non-illuminated surface is provided with apertures having a diameter not more than a wavelength of the incident light, forming a periodic surface topography, where an array of apertures has a strengthening characteristic causing resonance interaction between the incident light and surface plasmons on the surface; a second electrode disposed at a distance from the first metallic electrode; and a plurality of spheres disposed between the first metallic electrode and the second electrode corresponding to the array of apertures; wherein each sphere has a p-doped material first portion and an n-doped material second portion so that a p-n junction is formed at a junction between the first and the second portions, and each of the respective spheres is disposed within the apertures so that one of the first or the second portion is in electrical contact with the first metallic electrode, and the other of the first or the second portion is in electrical contact with the second electrode. The incident light resonates with the surface plasmons on the surface topography.
Furthermore, in order to increase light sensitivity, a light receiving element described in Patent Document 4 includes a semiconductor substrate, an n-type semiconductor layer disposed above the semiconductor substrate, an i-type semiconductor light absorbing layer disposed above an n-type contact layer and which absorbs light of a given wavelength band, and a p-type semiconductor layer disposed above the i-type semiconductor light absorbing layer, wherein the p-type semiconductor layer has periodically arrayed channels, holes, or columnar protrusions, includes a one dimensional or two dimensional photonic crystal part having a periodic refractive index distribution in a face intersecting the direction of incidence of light, and the photonic crystal part is formed so that at least a part of the incident light is even-number order diffracted light, and is distributed in a face of the light absorbing layer to be propagated.
[Patent Document 1]
JP Patent Kokai Publication No. JP-P2007-165359A
[Patent Document 2]
WO Pamphlet No. 2005/098966
[Patent Document 3]
JP Patent No. 3726887
[Patent Document 4]
JP Patent Kokai Publication No. JP-P2005-159002A